for an antibonding MO. Thus the Hückel mode is forbidden while the Möbius mode is allowed. One further relevant point is that the first organic correlation diagrams were in a 1961 publication on carbanion rearrangements. It had been noted that when an occupied molecular orbital becomes antibonding the reaction is inhibited and this phenomenon was correlated with a series of rearrangements. Relation of the Möbius–Hückel concept to the Woodward–Hoffmann methods Until 1969 there was no obvious relationship except that the two methods lead to the same predictions. As noted earlier, the Woodward–Hoffmann method requires symmetry. But in 1969 and 1970 a general formulation was published, namely, A ground-state pericyclic change is symmetry-allowed when the total number of (4q + 2)s and (4r)a components is odd. The 1969–1970 Woodward–Hoffmann general formulation is seen to be equivalent to the Zimmerman Möbius–Hückel concept. Thus each (4r)a component provides one plus-minus overlap in the cyclic array (i.e. an odd number) for 4n electrons. The (4q + 2)s component just makes certain that the number of electrons in symmetric bonds is 4n + 2. The equivalency of the more recent formulation of the Woodward–Hoffmann rules has been discussed. References Physical organic chemistry